The invention concerns an arrangement for determining relative atmospheric humidity by means of a capacitive atmospheric humidity sensor and associated sensor evaluation electronics, the capacitive atmospheric humidity sensor having a polymer layer as a dielectric and two electrically conductive electrodes.
Such an arrangement is known, for example, from DE 28 51 686, the dielectric properties of this polymer layer representing a function of the relative atmospheric humidity. The determination of the relative atmospheric humidity in this case takes place by measuring the capacitance between the two electrically conductive electrodes. An improvement in the speed and accuracy of the measurement, in particular when there is high relative humidity, is to be achieved in this case by heating up the polymer layer.
In the case of such capacitive atmospheric humidity sensors, it has been found that the polymer layers become contaminated by foreign substances in the course of the service life. In particular when there is frequent condensation (that means an atmospheric humidity of approximately 100%), and when it is cold, residues remain in the layer, so that the functional interrelationship between capacitance and atmospheric humidity is disturbed. Long-term instabilities or signal drift are thus the consequence. If these sensor elements are then heated up, for example dried out in an oven, it is also possible for the measured values to be stabilized again, with the foreign substances also being eliminated.
If these atmospheric humidity sensors are used for air-conditioning systems in buildings, they are usually serviced or replaced regularly. If, however, the atmospheric humidity sensors are used in air-conditioning systems of motor vehicles, they must operate under aggravated conditions without servicing, if possible over a period of years.
Thus there is a need for an arrangement for determining relative atmospheric humidity of the type stated at the beginning which operates satisfactorily without any additional servicing effort.
Other needs will become apparent upon a further reading of the following detailed description taken in conjunction with the drawings.
The aforementioned needs are fulfilled according to the invention by one of the electrically conductive, metallized electrodes being designed as a laminar heating resistor with at least two power terminals.
The arrangement according to the invention has the advantage that the capacitive atmospheric humidity sensor can be heated up outside the measuring periods in a simple way and at any time, so that any contaminations there may be can be removed by vaporization.
In the case of the arrangement according to the invention, it may be provided that the electrically conductive electrodes are arranged on both sides of the polymer layer, opposite one another, or on one side of the polymer layer, alongside one another.
It is particularly advantageous if the electrodes of the capacitive atmospheric humidity sensor consist of platinum, which has a relatively high resistivity and, in the form of a thin foil, is consequently particularly suited as a heating resistor. In principle, some other noble metal can also be used.
In the case of the preferred exemplary embodiment, it is therefore possible to make the foil layer thickness of the platinum electrodes so small that moisture can penetrate through.
For the heating up of the capacitive atmospheric humidity sensor, the platinum electrode designed as a heating resistor is supplied with current in specific time cycles, it having been found to be advantageous if this takes place each time that the arrangement is switched on.
In the case of another embodiment of the invention, it is proposed that the heating resistor is supplied with current for a short time only when the ambient temperature is below a fixed predetermined value when the device is switched on. Provided here for sensing the ambient temperature is a temperature sensor which is connected to the sensor evaluation electronics and emits an appropriate compensating variable for determining the exact relative atmospheric humidity by the sensor evaluation electronics.
A development of the invention is that the heating resistor is in each case supplied with current only after a fixed predetermined waiting time, which has proven to be particularly cost-effective and cost-saving.
Use of the arrangement according to the invention in an air-conditioning system, preferably of motor vehicles, is particularly advantageous, because such air-conditioning systems operate without servicing over a period of years.
These and other features and advantages of the invention will be apparent upon consideration of the following detailed description of the preferred embodiment of the invention, taken in conjunction with the appended drawings.